The recent announcement of the Advanced Photography System (APS) has brought a new era to photography. The system centers around a photographic film having a substantially transparent magnetic layer on the non-emulsion side of the film (referred to as the MOF layer). One or more longitudinal read/write tracks are provided in the MOF layer between the side edges of the image frame area and the film. Information such as film type, film speed, film exposure information, and information relevant to the processing and subsequent use (e.g., printing) of the exposed image frames may be prerecorded during production of the film cartridge. The prerecorded information useful for controlling camera operations can be read out in a camera and information related to exposure of an image frame can be recorded during camera use. Voice messages and/or sound associated with the photographed scene can also be recorded during camera use.
In the Advanced Photo System, the information is magnetically recorded on film in a standardized format that has been agreed to globally by the film and camera manufacturers. (See, for example, U.S. Pat. No. 5,130,745, issued Jul. 14, 1992, inventors Cloutier et al.).This format is illustrated in FIG. 1. Film 10 has a series of perforations 12 along one edge only. A first set of longitudinal magnetic tracks 14 and 16 are located between the perforations 12 on one side of image frames 18. A second set of longitudinal magnetic tracks 20 and 22 are located on the other side of frames 18 running along the unperforated edge of film 10.
The magnetically recorded tracks in the APS format are fixed in length and are relatively low in data density. Systems for storing and retrieving information from a physical medium, such as magnetic tape, must take steps to insure that the process can be done reliably. Often, physical defects in the medium, dust and dirt on the medium, or contamination from the medium can interfere with the storage or retrieval process. When this happens, long strings of data, called bursts, can be lost. Typically, powerful error correction codes and data interleaving are employed to correct for any error bursts that might occur. There are some magnetic data systems, however, such as the magnetically coated film systems described above, that cannot employ these methods, due to cost restrictions or small quantities of recorded data.
In the magnetically coated film system described above, the data tracks are of a fixed length. Current techniques for minimizing error problems in such systems are shown in FIGS. 2a-2c. Each of the shown techniques employ multiple recordings of the same data in a frame to achieve reliability in data recovery. FIG. 2a shows recording the same data three times within the fixed length of a track 28 of length L. Thus, data block 30 includes a preamble 32, data 34, and postamble 36. Each data block 30 has a length L/3. With such a format, for the data to be unrecoverable, all three recordings of the data would have to be corrupted, either by a single burst error extending through all three recording, two burst errors corrupting different segments of the three recordings, or three burst errors corrupting the three recordings.
FIG. 2b shows recording the data block 40 two times over the full length L of track 28'. Each data block 40 has a length L/2. This format has the very desirable feature of allowing substantially more data to be recorded in a track. However, a single short burst error that occurs in the middle of the track, could corrupt both of the data blocks, rendering the data set substantially unrecoverable. FIG. 2c shows recording a shorter data block 50 consecutively twice at the beginning of the fixed length track 28" leaving a space 52 at the end of the track. This format does little to reduce errors, since a single error burst that occurs bridging the data blocks 50 will corrupt the data set.